In order to get the 50 materials needed to craft the armor you need to defeat the boss up to tier #15. In order to gain the additional 50 materials needed to craft the plus version of the armor you need to defeat the boss up to tier #43.

This material can also be obtained by summoning the epic boss at the Summoning Stone. If the Boss is active, it will cost 11 gems; otherwise, it will cost 30 gems.